For lower-stakes COVID-19 checks, rapid antigen tests offer a quick and simple way to screen for SARS-CoV-2 at home. They can be particularly helpful if you know or suspect you've been exposed to the coronavirus but can't access professional testing (or wait for the results).
FDA-authorized at-home antigen diagnostic tests can detect active COVID-19 infections, including in asymptomatic individuals, in about 15 minutes. Overall, these tests are less sensitive than molecular diagnostic tests done in labs. Still, the results of at-home antigen testing can provide additional data—if not total peace of mind—regarding one's COVID-19 status, especially if you test frequently. Depending on your situation, it may make sense to have a few of these tests on hand.
Keep in mind that a negative result doesn't necessarily mean someone doesn't have COVID-19, and these tests aren't meant to be used as the sole method of diagnosis. “Antigen tests are a cheap and easy way to identify a person who may be contagious," said Dr. Matthew McCarthy, an associate professor of medicine at Weill Cornell Medical College. For people with no known COVID-19 exposures, “if you're going to Thanksgiving, and there's 20 people there and they're all fully vaccinated, you can do an antigen test before you go to make sure you're not bringing the virus into the party," he said, citing one potential use.
Taking a rapid antigen test might also be appropriate before interacting with someone who may be more vulnerable to contracting the virus. “Even people who are fully vaccinated may want to take one of these convenient at-home tests before going to spend time with Grandma, or whatever it may be," said Dr. Clare Rock, a clinical epidemiologist at Johns Hopkins University School of Medicine who runs a COVID-19 infection-control consultancy.
What's good about these tests—and what's not
At-home COVID-19 antigen tests are accessible and fast. Rather than waiting for an appointment (or ordering a mail-in kit and shipping a sample) and then waiting for the results of a molecular diagnostic test, with an at-home antigen test you can generally go from swab to result in as few as 15 minutes. The tests are generally easy to perform, and you can read the results manually (as with a home pregnancy test) or digitally (with an app).
COVID-19 antigen tests are less sensitive than molecular diagnostics. Unlike molecular diagnostic tests for COVID-19 that involve amplifying viral nucleic acids to the point where they can be more easily detected, antigen tests detect unamplified traces of virus and thus don't pick up small signals as readily. (Antigen tests are not to be confused with antibody tests, which are meant to detect antibodies produced to respond to the virus, and are not used for diagnostic purposes.)
Although at-home antigen tests are not fail-safe, even molecular diagnostics like the gold-standard PCR tests are not always accurate when it comes to detecting active COVID-19 cases, since results depend on, among other things, the timing of the test. If you're swabbed too soon after an exposure, you may test negative even if you are carrying the virus. You can also receive a positive PCR test result after you're no longer infectious.
Compared with molecular diagnostics for COVID-19, antigen tests are “not as sensitive if we're looking to understand is there any even minute piece of virus present," clinical epidemiologist Rock said, 'but they are very sensitive if we're looking to see is there a level of the virus that we have to be concerned that somebody could transmit to someone else."
How do different at-home COVID-19 antigen tests compare?
There are nine SARS-CoV-2 antigen tests from five companies now authorized by the FDA for emergency use at home. These are variations of Abbott's BinaxNow COVID-19 Antigen Self Test, Access Bio's CareStart COVID-19 Antigen Home Test, Ellume's COVID-19 Home Test, the InteliSwab COVID-19 Rapid Test from OraSure Technologies, and the Quidel QuickVue At-Home COVID-19 Test. Of these, the Abbott BinaxNow, Ellume COVID-19 Home, and Quidel QuickVue tests are currently available direct to consumers, without a prescription, in pharmacies and online.
The instructions that accompany all of these tests include a disclaimer that negative results may require additional testing for confirmation. For each of them, there's a small chance of false positive results, too.
“We typically recommend getting tested three to five days after exposure."
—Dr. Matthew McCarthy, Weill Cornell Medical College
The accuracy of an at-home antigen test depends in part on test sensitivity (the test's reported ability to detect a true positive), test specificity (its reported ability to detect a true negative), sample integrity (whether a swab contains enough sample or the swab solution is contaminated by, say, another pathogen), whether one follows the manufacturer's instructions exactly, the time since a person's last known or suspected exposure and/or their onset of symptoms, and one's viral load at the time of testing. (The tests are currently authorized for use on people as young as 2, provided any child's sample is obtained and processed by an adult.)
For a test to be considered for emergency use authorization, test makers must submit to the FDA clinical data demonstrating test sensitivity and specificity. Some independent studies have shown much lower sensitivity and specificity for some antigen tests, particularly when they're used on asymptomatic individuals. (There is one commercially available SARS-CoV-2 molecular diagnostic test currently authorized by the FDA for emergency use entirely at home, meaning you don't need to send a sample off to a lab for testing: the Lucira COVID-19 All-In-One Test Kit. It is slightly more sensitive—95.2%—compared with some FDA-authorized at-home antigen tests, and it provides results in 30 minutes. But it is currently unavailable on Lucira's website and at Amazon, where it was once sold.)
At the time of publication, at-home antigen tests were hard to find because a surge in COVID-19 cases caused a spike in demand for them. If you have trouble finding them online, call local pharmacies (these tests are often stocked behind the front counter).
How to take an at-home COVID-19 antigen test
The key to getting trustworthy antigen test results is testing frequently. “Serial testing boosts sensitivity," said Christoper Brooke, an infectious diseases expert at the University of Illinois at Urbana-Champaign. "The odds that you're going to test negative twice when you're infected are much lower than the odds that you're going to test negative once."
Rather than having a swab shoved up into the nasopharyngeal cavity, which you might encounter at a clinical testing site, the at-home antigen tests from Abbott, Ellume, and Quidel require a less-penetrative mid-nasal swab. Each test comes with specific instructions, which essentially require that you swab your nose, dip the swab into a solution, transfer some of the solution into a small reservoir, and wait for the result.
As is true for all COVID-19 diagnostics, including PCR tests, the timing of sample collection against the last known or suspected exposure and/or symptom onset is the biggest factor that can affect the accuracy of at-home antigen test results. This is why, for example, Abbott's BinaxNOW and Quidel's QuickVue test kits come with two tests, intended to be used between two and three days apart.
“The sensitivity of a test really depends on when you use it," said Daniel Larremore, an assistant professor of computer science at the University of Colorado Boulder who has modeled the effects of repeated population screening of asymptomatic people with molecular and antigen tests. An infected person's viral load changes over time. “When you reach a high enough viral load, antigens are going to be at a high enough concentration to be detected." Testing yourself the day after attending a party with someone who didn't know they had COVID-19 at the time is unlikely to be useful. “Twenty-four hours after exposure, no test is going to be positive," Larremore said. If you wait too long to test, you may miss peak antigen concentration, meaning you should see a fainter positive line if the test detects SARS-CoV-2 antigens in your sample.
“We typically recommend getting tested three to five days after exposure," said McCarthy of Weill Cornell. If you have COVID-19–like symptoms, there's no need to wait to test.
When to speak with a doctor
Consult a physician if you're unsure of or confused by the result of an at-home COVID-19 antigen test. Whether you should seek a confirmatory molecular test depends on your circumstances. One should treat a positive antigen test result as a true positive, said Larremore, particularly if additional factors—such as a potential exposure or the appearance of symptoms—support the result. This means isolating, alerting any contacts, and possibly seeking to confirm the result with a lab test. Seek medical treatment for symptoms as needed. According to Weill Cornell's McCarthy, following up on a negative antigen test result with a confirmatory molecular diagnostic may not be necessary in cases where someone has a low suspicion of having COVID-19 (for example, they are asymptomatic, fully vaccinated, and/or have no known exposures).
Getting a lab-performed PCR test is your best bet for an accurate COVID-19 diagnosis, but appointments can be difficult to get, and sometimes “it takes so long to get the results that they are useless," said Brooke of the University of Illinois. “Ideally everyone would have frequent PCR testing with rapid results reporting, but that's obviously not possible. Antigen tests are often the only really viable available choice, so they can play a really important role in increasing the frequency of testing, and the breadth of testing, across the population."
Original reference: At-Home COVID-19 Antigen Tests: What You Should Know | Wirecutter (nytimes.com)